Electrical erasable programmable memory transconductance testing

ABSTRACT

A test method determines if an array of a Flash EEPROM circuit has a bit cell with a transconductance (gm) that is deficient. The method preconditions all bit cells of the array to a particular programmed state and then determines whether any of the bit cells exhibit undesirable operating characteristics by reading each bit cell to determine whether its transconductance is less than desirable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to electrically erasable programmableread only memories and more particularly to a method of testingtransconductance of the cells of such memories.

2. Description of the Related Art

Electrically erasable programmable read only memories (EEPROM) such asflash memories are used for non volatile memory storage of informationfor data processing systems.

As dimensions for Flash EEPROM cells continue towards smallergeometries, degraded transconductance (gm) bitcells and/or the gmdegradation of the bitcells during use poses a reliability issue. Inparticular, degraded gm bitcells cause reliability and/or performanceissues and can result in a failure condition of the Flash EEPROM or theintegrated circuit that incorporates the Flash EEPROM.

Accordingly, there is a need for an improved method and apparatus forovercoming the problems in the art as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a block diagram of a memory circuit according to oneembodiment of the present invention.

FIG. 2 is a graph illustrating a bit cell with a bad transconductancecurve.

FIG. 3 is a graph illustrating aspects of a test for determining whethera bit cell has a bad transconductance curve according to one embodimentof the present invention.

FIG. 4 is a flow diagram for determining whether a memory circuit has abit cell with a bad transconductance curve according to one embodimentof the present invention.

FIG. 5 is a graph illustrating aspects of a test for determining whethera bit cell has a bad transconductance curve according to anotherembodiment of the present invention.

FIG. 6 is a flow diagram for determining whether a memory circuit has abit cell with a bad transconductance curve according to anotherembodiment of the present invention.

The use of the same reference symbols in different drawings indicatesidentical items unless otherwise noted. The Figures are not necessarilydrawn to scale.

DETAILED DESCRIPTION

The following sets forth a detailed description of a mode for carryingout the invention. The description is intended to be illustrative of theinvention and should not be taken to be limiting.

EEPROM cells such as flash memory cells can be characterized with use ofa transconductance (gm) curve. In particular, an EEPROM cell conducts acurrent based on the voltage applied to its word line (with the bit lineset to a particular value). A bit having bad transconductance can bedetermined with use of a transconductance curve for the bitcell, whereintransconductance of the bitcell which deviates from normal can providean indication of a bad bit cell. Accordingly, the embodiments of thepresent disclosure advantageously provide a method for detecting andsubstantially eliminating degraded gm bitcells in production testing(for repair or rejection) and can also provide a method for earlywarning eminent failure notification in customer applications.

In one embodiment, the method includes preconditioning and normalizingall bits to the same current and threshold voltage (Vt) pivot point. Inaddition, the method robustly identifies “bad” gm bitcells on all bitsbeing tested as the same pivot point is established. In one embodiment,all bits are preconditioned with a weak program operation with highgranularity to fix all bits at a predefined Ids and Vt level. Accordingto another aspect, the method determines whether the off-state leakageis low, noting that some bits may program faster than others, and that agross test may not measure the true gm of all bitcells. Furthermore, theembodiments of the present disclosure advantageously provide a methodand apparatus for enabling customer diagnostics of a diagnostic tool toflag a user of the presence a potential eminent failure condition hasbeen found. In response to detection of presence of a potential eminentfailure condition being found, appropriate measures can be carried outbefore the occurrence of an actual failure.

FIG. 1 is a memory circuit 101 that has an array 103 of EEPROM cells andcircuitry for determining whether any of the cells have a badtransconductance (gm) curve.

In the embodiment shown, memory array 103 includes four flash EEPROMcells 105, 107, 109, and 111. In one embodiment, the cells are floatinggate flash memory cells although other types of flash memory cells (e.g.nanocrystals, split gate flash, nitride based memories) may beimplemented. Also, other types of EEPROM cells may be implemented inarray 103. In the embodiment shown, array 103 includes 4 memory cellsfor illustrative purposes, but may include a greater number in otherembodiments.

Memory circuit 101 includes a row decoder 115 for selectively providingvoltages Vw from word line voltage control circuit 117 to the word linesWL0 and WL1 during memory operations. These words lines are connected tothe gates of the memory cells of array 103. Circuitry 101 also includesa column decoder and sense amplifier circuit 121 that includes senseamplifiers for coupling to the bit lines BL0 and BL1 for reading thecells of array 103 and circuitry for applying a Vdrain voltage fromdrain voltage control circuit 125 to the bit lines BL0 and BL1 duringmemory operations. Circuit 121 outputs the data read from the cells ondata lines. Circuit 101 also includes a current reference circuit 123for supplying a reference current Iref to a reference sense amplifier ofcircuit 121 for voltage reads of the cells circuit 121.

Circuit 101 includes a memory controller 113 for controlling theoperations of circuit 101 during its operation. Controller 113 controlswordline voltage control 117, reference circuit 123, drain voltagecontrol 125, and source control 127 to control the voltage and currentvalues provided by those circuits during memory operation. Memorycontroller 113 also provides control information to control theoperations of row decode circuit 115 and circuit 121 during memoryoperation. Controller 113 includes address, data, and control lines forreceiving address, data and control information from an external source(e.g. a processor) for performing memory operations. In the embodimentshown, one of the control lines includes a test line for signalingcontroller 113 to enter a test mode to determine whether a cell of array103 has a bad gm curve. Other memory circuits may have otherconfigurations in other embodiments.

Memory circuit 101 is coupled to a processor 150 via the address, data,and control lines. In one embodiment, processor 150 may be on the sameintegrated circuit or may be on a different integrated circuit.

FIG. 2 shows a graph illustrating problems of an array having a bad gmcurve for memory device operation. The X axis represents the voltage(Vwordline) applied to a word line of a bit cell during a read and the Yaxis represents the drain current (Idrain) (bit line current) of a bitcell during a memory read.

Plotted on FIG. 2 are the current-voltage (IV) curves for a normal bitcell of the least erased cell of an array, the IV curve for a readreference cell of an array, and the IV curve for a normal leastprogrammed cell of the array. The slope of these curves is thetransconductance (gm) of the cells. In an ideal memory array, the slopeof these three curves parallel each other such that regardless of theword line voltage, there will be drain current differentiation betweenthe three curves. For example, if a read voltage Vr is applied to thewordline, a drain current of Iref will be produced by the referencecell. A cell producing a higher drain current will be read as an erasedcell. A cell producing a lower drain current will be read as aprogrammed cell.

During erasing, erase voltages are applied to the cell until the cellproduces a higher current than Ev at the verify voltage Ve during a testread. During programming, programming voltages are applied to the celluntil the cell produces a lower drain current than PV at the programverify wordline voltage Vp during a test read. Such programming withmemory cells having normal gm curves allows for sufficientdifferentiation of drain currents between the erased cells andprogrammed cells for accurate reading of the cells during memoryoperation.

FIG. 2 also illustrates an IV curve for an erased cell with a “bad” gmcurve. In the embodiment of FIG. 2, the slope of the IV curve is lessthan that for a normal curve. In the example shown, the bad cell may beerased such that its drain current at wordline voltage Ve is higher thanEV thereby testing as an erased cell. However, because its gm curve islower, the drain current at Vr is less than that of the normal leasterased cell (illustrated as Idelta). The less differential from thereference cell at Vr means that there is a greater likelihood that thatcell may provide for a false read. In addition, a low gm curve maypresent similar problems in programming in that a programmed cell mayhave a smaller drain current than program value PV at wordline voltageVp but still may not provide a sufficiently low drain current at voltageVr.

In some embodiments, a lower gm curve of a cell may be indicative of amanufacture defect in the cell. Furthermore, the gm of a flash cell maydegrade over time due to electrical stress on the cell. Thus, a cellwhich may provide acceptable test results in drain current at voltagesVe, Vr, and Vp during manufacture testing, may fail over time due tosuch stress. Accordingly, it is desirable to determine whether a cellhas a bad gm during manufacture even if such a cell may pass read testsduring manufacture.

FIG. 3 is a graph illustrating concepts of a method for determining acell with a bad gm curve according to one embodiment of the presentinvention. In the embodiment shown, all cells of an array are initiallyprogrammed such that they produce particular a drain current (e.g.2Iref1) at a particular higher wordline voltage Vwl1 (point PT1 in FIG.3).

In one embodiment, this initial programming is performed by a “weak”programming operation where a reduced gate voltage (word line) and areduced drain voltage is applied to the cell. Such gate and drainvoltages are reduced from values utilized during programming the cellfor normal operation. For example, for a memory array where a gatevoltage of 9.3 V and a drain voltage of 5 V are applied during normaloperations, a gate voltage of 5 V and a drain voltage of 4 V may beapplied for weak programming for the initial programming. Other voltagesmay be used in other embodiments. In one embodiment, weak programming isused to achieve a “tighter” distribution of wordline voltages at a draincurrent of 2Iref1 than could be obtained with regular programming. Thistighter distribution provides for a more accurate test in finding cellswith bad gm. In one embodiment, each cell is programmed such that eachcell produces a drain current of 2Iref1 and the world line voltage ofthe most programmed cell is within 500 mv of the wordline voltage of theleast programmed cell.

FIG. 3 illustrates the IV curve for a normal cell (normal gm) that hasbeen programmed to point PT1. Also illustrated in FIG. 3 is an IV curvefor a cell with a bad gm curve. In the embodiment shown, the cell with abad gm curve will have a higher drain current at a lower wordlinevoltage than voltage Vwl1.

In one embodiment, Iref1=12 micro amps, 2 Iref1=24 micro amps, and ¼Iref1=3 micro amps. Other current values, including other ratios of Irefmay be used in other embodiments.

To determine whether a particular cell has a bad gm, the cell is readwith a wordline voltage of Vrg which is less than Vwl1. If the cell hasa higher drain current than a particular value (e.g. ¼ Iref1), then thecell is considered to have a bad gm curve. If a cell has a drain currentof less than ¼ Iref1, then the cell is considered to have an acceptablegm.

FIG. 4 is a flow diagram of one method for determining whether a memorycircuit includes a bit cell with a bad gm curve. In operation 401, thememory circuit enters a diagnostic mode. In one embodiment a diagnosticmode is entered when memory controller 113 receives a signal on the testline to enter the diagnostic mode. In other embodiments, memory cellenters a diagnostic mode when it receives commands from a processor(e.g. 150) or external tester.

In operation 403, all bits of the array are erased. In one example, theerase operation may be performed on a block by block basis for an arraywith multiple blocks.

In operation 405, the controller 113 sets the wordline control circuit117 to provide the appropriate word line voltage, the reference circuit123 to provide the appropriate reference current value, and the draincontrol circuit to provide the appropriate drain voltage for weakprogramming of each cell. Each cell is then programmed on a bit by bitbasis in operation 407 to the state such that each cell produces aparticular drain current (e.g. 2Iref1 in FIG. 3) by applying a wordlinevoltage within a particular range (e.g. Vwl1 in FIG. 3). In oneembodiment, programming is performed by applying programming pulses to acell followed by a verify read to see if the cell is programmed withinthe desired read characteristics.

After all of the cells have been programmed to the desired programmingstate in operation 407, memory controller 113, in operation 409, resetsthe Iref and Vwordline values for the detection of a cell with a bad gmcurve. In the embodiment of FIG. 3, the reference current to thereference cell is set at ¼ Iref1 and the word line voltage is set toVrg. These values are set to less than those in program operation 407 totest the cell at different values in the gm curve.

In operation 411, all of the bits are read to determine if any of thecells produce a drain current that is greater than the threshold current¼ Iref1 (the reference current provided to the reference cell). In oneembodiment, this is performed by reading each cell with its wordlinevoltage set to Vrg and comparing the drain current of the cell with thedrain current of the reference cell. A drain current of higher than thereference cell drain current indicates that the cell is in the erasedstate. If in 413 a cell is read as having a higher drain current than ¼Iref1, then in operation 417, the memory circuit is deemed to fail inthat at least one cell has a bad gm curve. If none of the cells have ahigher drain current than ¼ Iref1, then the memory circuit is deemed topass the gm test in operation 415. In operation 421, memory circuit 101exits test mode.

In the embodiment of FIG. 4, if any cell has a bad gm, then the memorycircuit is deemed defective. In one embodiment, the integrated circuitincluding the memory circuit is discarded. However, it otherembodiments, the cell or cells with defective gms may be identifiedwherein such calls may be replaced with redundant memory cells of thecircuit. Still in other embodiments, cells that are defective may bemarked as unusable for storage.

In one embodiment, the memory controller is controlled by processor 150via the control lines for performing the operations of FIG. 4. Inanother, memory circuit 101 is coupled to a tester during testing forcontrolling memory controller 113 during testing.

Utilizing the above method for testing a memory array provides for amethod for testing the array for cells with bad transconductance curvesduring manufacture to determine whether any of the cells are defectivein a memory array. Accordingly, accurate testing may be performed beforesending a defective part to a customer.

In one embodiment, memory controller 113 includes registers for storingwhether the part failed or passed the gm test. In some embodiments,controller 113 includes registers for storing the addresses of thefailed cells.

FIG. 5 is a graph setting forth a second embodiment for determiningwhether a cell has a bad gm curve. Like the embodiment of FIG. 3, inthis embodiment, all of the cells are programmed to a particularprogrammed state (e.g. to programmed state PT1 in FIG. 3). However, inthis embodiment, a cell is determined to be defective if the minimumvoltage applied to a wordline to make the cell produce a particulardrain current is less than a particular voltage.

In the embodiment of FIG. 5, the reference current is set at aparticular threshold (e.g. at ¼ Iref1). Iref1 is the reference currentprovided to the reference cell for reading the reference cell duringnormal read operations. A cell is deemed to have a defective gm if thewordline voltage applied to the cell for the cell to produce theparticular drain current is less than a particular voltage value. Such acondition after the cell has been preconditioned to a particularprogrammed state (e.g. state PT1 in FIG. 3) would indicate that the gmof that cell is defective.

The average gm for cells of an array may vary due to manufacturingconditions of the wafer that produced the memory array. A variance inall of the gm curves for an array in the same way may not necessarilycause a failure in the part in that the reference cell would also havethe same variance in its gm as well.

Accordingly, in the embodiment of FIG. 5, the particular voltagethreshold between a good gm and a bad gm may be based on thecharacteristics of the device to account for variations in itsmanufacture. In one embodiment, the particular minimum wordline voltagefor determining a bad gm would be based on an average wordline voltageof the array for producing a particular drain current. In the embodimentof FIG. 5, an indication of the average wordline voltage (Vw ave) of thecell is determined. A cell is considered to have a bad gm if itswordline voltage to produce a particular drain current is outside apredetermined range of the average. In one embodiment, a cell would bedeemed to have a bad gm if its wordline voltage is greater than 250 millvolts from the average wordline voltage. Other ranges may be used inother embodiments.

Providing a system where the test for determining a bad gm is based onaverage operating parameters of an array provides for a test that canaccount for variations in manufacturing conditions. Accordingly, such atest may yield less false positives for gm defect detectability.

FIG. 6 is a flow chart illustrating a method of implementing theembodiment of FIG. 5. Operations 601, 603, 605, and 607 forpreconditioning the cells to particular programmed state (e.g. PT1 ofFIG. 3) are similar to operations 401, 403, 405, and 407 of theembodiment of FIG. 4.

In operation 609, the current Iref to the reference cell is set to avalue for detecting cells with a bad gm. In one embodiment, thereference current is set to ¼ Iref1.

In operation 611, the average wordline voltage of the cells in the arrayfor producing a drain current of ¼ Iref1 is determined. In oneembodiment, the average wordline voltage is determined by measuring thewordline voltage for each cell. However, in other embodiments, theaverage wordline voltage is determined by measuring the average wordlinevoltage for a subset of the memory array (e.g. for a page of cells ofthe memory array). By using a subset of memory cells for wordlinevoltage determination, the time needed to determine the average wordlinevoltage is reduced.

In operation 611, the word line voltage for the least programmed cell isdetermined. In one embodiment, this operation is performed by settingeach wordline at 0 volts and individually applying an increasing voltageto each line until a drain current is detected on a bit line as beingabove ¼ Iref1. For example, 0.25 volts is initially applied sequentiallyto each word line. This voltage is increased by 0.25 volts until a draincurrent of greater than ¼ Iref1 is detected. If the first detectedwordline voltage for the least programmed cell is within the acceptablerange in decision 613, then in operation 615, the memory circuit isdeemed to have passed. If the first detected wordline voltage for theleast programmed cell is outside the range, then the part is deemed tohave failed the gm detect ability test in operation 617. The test modeis exited in operation 621. In one embodiment, the acceptable range iswithin 250 millivolts of the average wordline voltage.

In another embodiment, determining whether an array includes a cellhaving a wordline voltage outside the acceptable range can be performedby setting the word line voltage to a voltage that is immediatelyoutside the acceptable range and determining if any bit cells on thatwordline has a drain current of greater than ¼ Iref1. If no cell on anywordline is detected have a drain current of greater then ¼ Iref1 atthat voltage, then the memory circuit is determined to pass.

By now it should be appreciated that there has been provided a methodfor detecting degraded transconductance in bit cells of a memory arraycomprises pre-conditioning, reading, and determining as follows. Themethod includes performing a pre-conditioning program operation on thebit cells with use of a pre-conditioning reference current andpre-conditioning bit line and word line voltages to render the bit cellsprogrammed. The programmed bit cells collectively have apre-conditioning threshold voltage distribution width less than athreshold voltage distribution width resulting from a normal programoperation on the bit cells. The method also includes reading all bitcells with use of a verification reference current and verification wordline voltage, the verification reference current and word line voltagebeing different from the pre-conditioning reference current and wordline voltage, respectively. In addition, the method includes determiningwhether any read bit cell is erased, as opposed to being programmed,wherein (i) responsive to any bit cell being erased, then at least onebit cell of the memory array is detected as having degradedtransconductance, and (ii) responsive to no bit cell being erased, thenno bit cell of the memory array is detected as having degradedtransconductance.

The pre-conditioning program operation can comprise programming the bitcells on a bit by bit basis with predefined values of increasedreference current (Iref), reduced bit line or drain voltage (Vbl), andreduced word line or gate voltage (Vwl) with respect to correspondingvalues for the normal program operation. In one embodiment, thepredefined values for the pre-conditioning program operation comprise areference current of 24 microamps, a bit line voltage of 4.0 volts, anda word line voltage of 5.0 volts, and wherein the corresponding valuesfor the normal program operation comprise a reference current of 12microamps, a bit line voltage of 5.0 volts, and a word line voltage of9.3 volts.

According to another embodiment, the verification reference current andverification word line voltage comprise degraded transconductance detectvalues. In addition, prior to reading all bit cells at degradedtransconductance detect values, the method further comprisesestablishing degraded transconductance detect values by setting a valuefor a reduced reference current (Iref) and setting a value for adegraded transconductance detect word line voltage (Vwl). In a furtherembodiment, the reduced reference current (Iref) comprises one-quarterof the normal program operation reference current (Iref) and wherein thedegraded transconductance detect word line voltage (Vwl) comprises avoltage (i) below the corresponding word line voltage for the normalpre-conditioned programmed bit cells and (ii) above the word linevoltage for a bit cell of degraded transconductance.

According to yet another embodiment, the method includes wherein theverification reference current (Iref) comprises one-quarter of a normalprogram operation reference current (Iref) and wherein the value for theverification word line voltage (Vwl) comprises a voltage equal to (i) anaverage word line voltage for all bit cells minus (ii) a degradedtransconductance detection limit. In addition, the degradedtransconductance detection limit comprises one-half of thepre-conditioning threshold voltage distribution width. Still further,the degraded transconductance detection limit can comprise 250millivolts.

In another embodiment, the verification reference current (Iref)comprises one-quarter of a normal program operation reference current(Iref) and wherein the value for the verification word line voltage(Vwl) is selected across a range of voltages from an average voltage toa voltage corresponding to a least programmed bit. The method furthercomprises instead of determining whether any read bit cell is erased,determining whether the lowest value of word line voltage (Vwl) for theleast programmed bit occurs at a voltage above a Vwl_lower_limit value.Responsive to the lowest value of word line voltage (Vwl) being lowerthan the Vwl_lower_limit value, then at least one bit cell of the memoryarray is detected as having degraded transconductance. Responsive to thelowest value of word line voltage (Vwl) being greater than or equal tothe Vwl_lower_limit value, then no bit cell of the memory array isdetected as having degraded transconductance. In addition, the leastprogrammed bit can represent the bit cell having a lowest value of wordline voltage (Vwl) in the pre-conditioning threshold voltagedistribution.

In a further embodiment, the Vwl_lower_limit value can equal (i) anaverage word line voltage for all bit cells minus (ii) a degradedtransconductance detection limit. In addition, the degradedtransconductance detection limit can comprise one-half of thepre-conditioning threshold voltage distribution width. Still further, inone embodiment, the degraded transconductance detection limit cancomprise 250 millivolts.

According to another embodiment, the step of determining whether any bitcell read is erased further comprises (i) responsive to any bit cellbeing erased, then failing the memory array as having detected degradedtransconductance in at least one bit cell of the memory array, and (ii)responsive to no bit cell being erased, then passing the memory array asnot having detected degraded transconductance in any bit cell.

In another embodiment, a method for detecting degraded transconductancein bit cells of a memory array comprises: performing a pre-conditioningprogram operation on the bit cells with use of a pre-conditioningreference current and pre-conditioning bit line and word line voltagesto render the bit cells programmed, the programmed bit cellscollectively having a pre-conditioning threshold voltage distributionwidth less than a threshold voltage distribution width resulting from anormal program operation on the bit cells; reading all bit cells withuse of a verification reference current and verification word linevoltage, the verification reference current and word line voltage beingdifferent from the pre-conditioning reference current and word linevoltage, respectively, wherein the verification reference current (Iref)comprises one-quarter of a normal program operation reference current(Iref); and performing one selected from the group consisting of (i)determining whether any read bit cell is erased, as opposed to beingprogrammed, wherein (a) responsive to any bit cell being erased, then atleast one bit cell of the memory array is detected as having degradedtransconductance, and (b) responsive to no bit cell being erased, thenno bit cell of the memory array is detected as having degradedtransconductance, and (ii) determining whether the lowest value of wordline voltage (Vwl) for the least programmed bit occurs at a voltageabove a Vwl_lower_limit value, wherein (a) responsive to the lowestvalue of word line voltage (Vwl) being lower than the Vwl_lower_limitvalue, then at least one bit cell of the memory array is detected ashaving degraded transconductance, and (b) responsive to the lowest valueof word line voltage (Vwl) being greater than or equal to theVwl_lower_limit value, then no bit cell of the memory array is detectedas having degraded transconductance. The value for the verification wordline voltage (Vwl) is selected across a range of voltages from a minimumvoltage to a voltage corresponding to a least programmed bit. Inaddition, the method comprises further wherein (iii) responsive to anybit cell being erased or the lowest value of word line voltage (Vwl)being lower than the Vwl_lower_limit value, then failing the memoryarray as having detected degraded transconductance in at least one bitcell of the memory array, and (iv) responsive to no bit cell beingerased or the lowest value of word line voltage (Vwl) being greater thanor equal to the Vwl_lower_limit value, then passing the memory array asnot having detected degraded transconductance in any bit cell. Stillfurther, the method includes wherein the pre-conditioning programoperation comprises programming the bit cells on a bit by bit basis withpredefined values of increased reference current (Iref), reduced bitline voltage (Vbl), and reduced word line voltage (Vwl) with respect tocorresponding values for the normal program operation.

In yet another embodiment, a method for detecting degradedtransconductance in bit cells of a memory array comprises: performing apre-conditioning program operation on the bit cells with use of apre-conditioning reference current and pre-conditioning bit line andword line voltages to render the bit cells programmed, the programmedbit cells collectively having a pre-conditioning threshold voltagedistribution width less than a threshold voltage distribution widthresulting from a normal program operation on the bit cells, wherein thepre-conditioning program operation comprises programming the bit cellson a bit by bit basis with predefined values of increased referencecurrent (Iref), reduced bit line voltage (Vbl), and reduced word linevoltage (Vwl) with respect to corresponding values for the normalprogram operation; reading all bit cells with use of a verificationreference current and verification word line voltage, the verificationreference current and word line voltage being different from thepre-conditioning reference current and word line voltage, respectively,wherein the verification reference current (Iref) comprises one-quarterof a normal program operation reference current (Iref) and wherein thevalue for the verification word line voltage (Vwl) is selected across arange of voltages from a minimum voltage to a voltage corresponding to aleast programmed bit; and performing one selected from the groupconsisting of (i) determining whether any read bit cell is erased, asopposed to being programmed, wherein (a) responsive to any bit cellbeing erased, then at least one bit cell of the memory array is detectedas having degraded transconductance, and (b) responsive to no bit cellbeing erased, then no bit cell of the memory array is detected as havingdegraded transconductance, and (ii) determining whether the lowest valueof word line voltage (Vwl) for the least programmed bit occurs at avoltage above a Vwl_lower_limit value, wherein (a) responsive to thelowest value of word line voltage (Vwl) being lower than theVwl_lower_limit value, then at least one bit cell of the memory array isdetected as having degraded transconductance, and (b) responsive to thelowest value of word line voltage (Vwl) being greater than or equal tothe Vwl_lower_limit value, then no bit cell of the memory array isdetected as having degraded transconductance.

In other embodiments, other memory circuits may have otherconfigurations. Further, the memory circuits may be implemented withother circuitry of an integrated circuit such as a microprocessor ormicrocontroller. Still in other embodiments, other methodologies caninclude determining whether a cell has defective gm using the cell's gmcurve.

Because the apparatus implementing the present invention is, for themost part, composed of electronic components and circuits known to thoseskilled in the art, circuit details will not be explained in any greaterextent than that considered necessary as illustrated above, for theunderstanding and appreciation of the underlying concepts of the presentinvention and in order not to obfuscate or distract from the teachingsof the present invention.

Although the invention has been described with respect to specificconductivity types or polarity of potentials, skilled artisansappreciated that conductivity types and polarities of potentials may bereversed.

As discussed herein, in one embodiment, with respect to a bit cell thatis in a programmed state, programmed means that the bit cell providesless current than the reference current. With respect to a bit cell thathas been erased, erased means that the bit cell provides a greatercurrent than the reference current. Furthermore, while variousembodiments as disclosed herein include preconditioning the array usinga higher Iref and then looking for degraded gm bits with lower Iref, itis to be understood that a similar objective can be met by swapping theIref values. In other words, the method can precondition the array (or asubset of the array) with lower Iref and then look for degraded gm bitsof the array (or the subset of the array) with higher Iref.

While particular embodiments of the present invention have been shownand described, it will be recognized to those skilled in the art that,based upon the teachings herein, further changes and modifications maybe made without departing from this invention and its broader aspects,and thus, the appended claims are to encompass within their scope allsuch changes and modifications as are within the true spirit and scopeof this invention.

1. A method for detecting degraded transconductance in bit cells of amemory array comprising: performing a pre-conditioning program operationon the bit cells with use of a pre-conditioning reference current andpre-conditioning bit line and word line voltages to render the bit cellsprogrammed, the programmed bit cells collectively having apre-conditioning threshold voltage distribution width less than athreshold voltage distribution width resulting from a normal programoperation on the bit cells; reading all bit cells with use of averification reference current and verification word line voltage, theverification reference current and word line voltage being differentfrom the pre-conditioning reference current and word line voltage,respectively; and determining whether any read bit cell is erased, asopposed to being programmed, wherein (i) responsive to any bit cellbeing erased, then at least one bit cell of the memory array is detectedas having degraded transconductance, and (ii) responsive to no bit cellbeing erased, then no bit cell of the memory array is detected as havingdegraded transconductance.
 2. The method of claim 1, wherein thepre-conditioning program operation comprises programming the bit cellson a bit by bit basis with predefined values of increased referencecurrent (Iref), reduced bit line voltage (Vbl), and reduced word linevoltage (Vwl) with respect to corresponding values for the normalprogram operation.
 3. The method of claim 2, wherein the predefinedvalues for the pre-conditioning program operation comprise a referencecurrent of 24 microamps, a bit line voltage of 4.0 volts, and a wordline voltage of 5.0 volts, and wherein the corresponding values for thenormal program operation comprise a reference current of 12 microamps, abit line voltage of 5.0 volts, and a word line voltage of 9.3 volts. 4.The method of claim 1, wherein the verification reference current andverification word line voltage comprise degraded transconductance detectvalues.
 5. The method of claim 4, wherein prior to reading all bit cellsat degraded transconductance detect values, the method furthercomprising: establishing degraded transconductance detect values bysetting a value for a reduced reference current (Iref) and setting avalue for a degraded transconductance detect word line voltage (Vwl). 6.The method of claim 5, wherein the reduced reference current (Iref)comprises one-quarter of the normal program operation reference current(Iref) and wherein the degraded transconductance detect word linevoltage (Vwl) comprises a voltage (i) below the corresponding word linevoltage for the normal pre-conditioned programmed bit cells and (ii)above the word line voltage for a bit cell of degraded transconductance.7. The method of claim 1, wherein the verification reference current(Iref) comprises one-quarter of a normal program operation referencecurrent (Iref) and wherein the value for the verification word linevoltage (Vwl) comprises a voltage equal to (i) an average word linevoltage for all bit cells minus (ii) a degraded transconductancedetection limit.
 8. The method of claim 7, wherein the degradedtransconductance detection limit comprises one-half of thepre-conditioning threshold voltage distribution width.
 9. The method ofclaim 8, wherein the degraded transconductance detection limit comprises250 millivolts.
 10. The method of claim 1, wherein the verificationreference current (Iref) comprises one-quarter of a normal programoperation reference current (Iref) and wherein the value for theverification word line voltage (Vwl) is selected across a range ofvoltages from an average voltage to a voltage corresponding to a leastprogrammed bit, the method further comprising: instead of determiningwhether any read bit cell is erased, determining whether the lowestvalue of word line voltage (Vwl) for the least programmed bit occurs ata voltage above a Vwl_lower_limit value, wherein (i) responsive to thelowest value of word line voltage (Vwl) being lower than theVwl_lower_limit value, then at least one bit cell of the memory array isdetected as having degraded transconductance, and (ii) responsive to thelowest value of word line voltage (Vwl) being greater than or equal tothe Vwl_lower_limit value, then no bit cell of the memory array isdetected as having degraded transconductance.
 11. The method of claim10, wherein the least programmed bit represents the bit cell having alowest value of word line voltage (Vwl) in the pre-conditioningthreshold voltage distribution.
 12. The method of claim 10, wherein theVwl_lower_limit value is equal to (i) an average word line voltage forall bit cells minus (ii) a degraded transconductance detection limit.13. The method of claim 12, wherein the degraded transconductancedetection limit comprises one-half of the pre-conditioning thresholdvoltage distribution width.
 14. The method of claim 13, wherein thedegraded transconductance detection limit comprises 250 millivolts. 15.The method of claim 1, wherein determining whether any bit cell read iserased further comprises (i) responsive to any bit cell being erased,then failing the memory array as having detected degradedtransconductance in at least one bit cell of the memory array, and (ii)responsive to no bit cell being erased, then passing the memory array asnot having detected degraded transconductance in any bit cell.
 16. Amethod for detecting degraded transconductance in bit cells of a memoryarray comprising: performing a pre-conditioning program operation on thebit cells with use of a pre-conditioning reference current andpre-conditioning bit line and word line voltages to render the bit cellsprogrammed, the programmed bit cells collectively having apre-conditioning threshold voltage distribution width less than athreshold voltage distribution width resulting from a normal programoperation on the bit cells; reading all bit cells with use of averification reference current and verification word line voltage, theverification reference current and word line voltage being differentfrom the pre-conditioning reference current and word line voltage,respectively, wherein the verification reference current (Iref)comprises one-quarter of a normal program operation reference current(Iref); and performing one selected from the group consisting of (i)determining whether any read bit cell is erased, as opposed to beingprogrammed, wherein (a) responsive to any bit cell being erased, then atleast one bit cell of the memory array is detected as having degradedtransconductance, and (b) responsive to no bit cell being erased, thenno bit cell of the memory array is detected as having degradedtransconductance, and (ii) determining whether the lowest value of wordline voltage (Vwl) for the least programmed bit occurs at a voltageabove a Vwl_lower_limit value, wherein (a) responsive to the lowestvalue of word line voltage (Vwl) being lower than the Vwl_lower_limitvalue, then at least one bit cell of the memory array is detected ashaving degraded transconductance, and (b) responsive to the lowest valueof word line voltage (Vwl) being greater than or equal to theVwl_lower_limit value, then no bit cell of the memory array is detectedas having degraded transconductance.
 17. The method of claim 16, whereinthe value for the verification word line voltage (Vwl) is selectedacross a range of voltages from a minimum voltage to a voltagecorresponding to a least programmed bit.
 18. The method of claim 16,further wherein (iii) responsive to any bit cell being erased or thelowest value of word line voltage (Vwl) being lower than theVwl_lower_limit value, then failing the memory array as having detecteddegraded transconductance in at least one bit cell of the memory array,and (iv) responsive to no bit cell being erased or the lowest value ofword line voltage (Vwl) being greater than or equal to theVwl_lower_limit value, then passing the memory array as not havingdetected degraded transconductance in any bit cell.
 19. The method ofclaim 16, wherein the pre-conditioning program operation comprisesprogramming the bit cells on a bit by bit basis with predefined valuesof increased reference current (Iref), reduced bit line voltage (Vbl),and reduced word line voltage (Vwl) with respect to corresponding valuesfor the normal program operation.
 20. A method for detecting degradedtransconductance in bit cells of a memory array comprising: performing apre-conditioning program operation on the bit cells with use of apre-conditioning reference current and pre-conditioning bit line andword line voltages to render the bit cells programmed, the programmedbit cells collectively having a pre-conditioning threshold voltagedistribution width less than a threshold voltage distribution widthresulting from a normal program operation on the bit cells, wherein thepre-conditioning program operation comprises programming the bit cellson a bit by bit basis with predefined values of increased referencecurrent (Iref), reduced bit line voltage (Vbl), and reduced word linevoltage (Vwl) with respect to corresponding values for the normalprogram operation; reading all bit cells with use of a verificationreference current and verification word line voltage, the verificationreference current and word line voltage being different from thepre-conditioning reference current and word line voltage, respectively,wherein the verification reference current (Iref) comprises one-quarterof a normal program operation reference current (Iref) and wherein thevalue for the verification word line voltage (Vwl) is selected across arange of voltages from a minimum voltage to a voltage corresponding to aleast programmed bit; and performing one selected from the groupconsisting of (i) determining whether any read bit cell is erased, asopposed to being programmed, wherein (a) responsive to any bit cellbeing erased, then at least one bit cell of the memory array is detectedas having degraded transconductance, and (b) responsive to no bit cellbeing erased, then no bit cell of the memory array is detected as havingdegraded transconductance, and (ii) determining whether the lowest valueof word line voltage (Vwl) for the least programmed bit occurs at avoltage above a Vwl_lower_limit value, wherein (a) responsive to thelowest value of word line voltage (Vwl) being lower than theVwl_lower_limit value, then at least one bit cell of the memory array isdetected as having degraded transconductance, and (b) responsive to thelowest value of word line voltage (Vwl) being greater than or equal tothe Vwl_lower_limit value, then no bit cell of the memory array isdetected as having degraded transconductance.